The present invention relates to gas discharge display panels and, more particularly, to such display panels which operate in a D.C. mode.
Gas discharge panels in which two orthogonal sets of conductors sandwiched an ionizable gas are well known in the art. In such devices, a potential applied to one of the anodes and one of the cathodes will result in the excitation of the gas at the intersection of those electrodes, and the resulting gas discharge will emit a visible light.
In A.C. gas discharge panels, the electrodes are isolated from the gas by a dielectric. During each half cycle of the A.C. excitation signal, a surface wall charge will build up on the surface of the dielectric in contact with the gas, and this wall charge will oppose the drive signal. This is advantageous in an A.C. display panel since the surface wall charge will rapidly extinguish the gas discharge and assist in breaking down the gas during the next half cycle of the A.C. signal. Since each break-down during each half cycle of operation produces light emission from the selected cell or cells, a flicker-free display can be achieved by operating the display at a relatively high frequency, e.g., 30-40 kilocycles. A disadvantage of A.C. display panels is that the A.C. drive signal generation systems are quite expensive and the light output is sometimes unsatisfactory.
An alternative to the A.C. gas discharge panel is a D.C. panel which, like the A.C. panel, consists of two sets of orthogonally arranged conductors sandwiching an ionizable gas. In conventional D.C. operated gas discharge panels, the metal electrodes are in direct contact with the discharge. Therefore, the cathodes are constantly being bombarded by gas ions during D.C. operation. These gas ions may have sufficient energy to sputter atoms from the cathode surface. Many of the sputtered atoms will be deflected back to the cathode surface by collisions with the gas ions, but some will escape collisions with the gas ions and be deposited on some other surface within the device. This sputtering phenomenon will result in a decrease in the usable life of the device and it will also make cell switching more difficult.
Certain proposals have been made for protecting the cathodes in a D.C. panel from sputtering, but none have proven satisfactory. If a protective layer overlying the electrodes is employed, such a layer cannot be conductive without shorting out adjacent cathodes. It also cannot be a dielectric protective layer, since a dielectric will isolate the gas discharge cell from the D.C. excitation voltage. In contrast to the A.C. panel, in which a surface will charge build-up is desirable in order to aid in extinguishing the discharge and cause break-down during the next half cycle, a surface wall charge build-up in a D.C. operated panel will decrease the effective potential applied to the gas until the net voltage falls below the minimum required to sustain a gas discharge, at which time the cell will turn "off."
A somewhat similar problem has been recognized in A.C. discharge panels. In A.C. panels, the dielectric layer overlying the electrodes and isolating them from the discharge gas can become degraded due to ion bombardment from the discharge and, therefore, refractory oxide coverings for the dielectric layer have been proposed. However, the secondary emission characteristics of a refractory oxide such as magnesium oxide will increase under operating conditions, resulting in lowering the panel operation margin (V.sub.s max. - V.sub.s min.) by decreasing V.sub.s max. (where V.sub.s is the potential required to sustain gas discharge). In U.S. Pat. No. 4,053,804, assigned to the same assignee as the present application, this inventor has disclosed a technique for solving this problem in A.C. discharge panels. The technique comprises depositing over the dielectric layer in the A.C. panel a protective covering of MgO, which may be approximately 2,000 A thick, and then depositing over the MgO layer a further layer of MgO doped to a level of 5% gold. The gold-doped layer is relatively thin, on the order of 200 A. The gold doping will sufficiently reduce the secondary emission characteristics of the MgO to provide a relatively constant operating margin by substantially reducing the decrease in V.sub.s max.
In an A.C. discharge panel, as described above, it is important to have a charge build up on opposite surfaces of the discharge cell, the charge build-up having a polarity which is opposite the polarity of the A.C. excitation signal, to aid in promptly extinguishing the discharge and in causing gas breakdown during the following half cycle of operation. If the MgO protective layer is doped with a substantial amount of gold, the surface charge will be permitted to migrate into the MgO layer and the cell will not operate satisfactorily. Thus, the upper surface layer of the MgO protective layer is doped with a small amount of gold, e.g., 200 A. This will substantially lower the secondary emission characteristics but will not permit the surface wall charge build-up to dissipate.
Such a sputtering protection technique would not be acceptable in correcting cathode sputtering in a D.C. discharge panel, since any surface charge build-up is undesirable in D.C. operation.